US journalist Steven Sotloff is pictured in this undated handout photo obtained by Reuters.
Photograph: Reuters

Islamic militants released a video on Tuesday that purported to show the beheading of a second captive American journalist, Steven Sotloff, which ended with a chilling warning that a British hostage would be the next to die.

In a video, entitled A Second Message to America, a masked man is shown carrying out the decapitation of Sotloff, whose life had earlier been threatened in a film that showed the murder of another American journalist, James Foley.

The video, released by the Islamic State (Isis) on Tuesday, features a voiceover delivered by a British jihadi with a ­distinctive London accent, apparently the same man who was filmed beheading Foley two weeks ago. It ends with the killer threatening another hostage, identified as a Briton.

US officials said intelligence experts had begun work on establishing the authenticity of the video, which runs for two minutes and 50 seconds. The British prime minister David Cameron condemned the killing as a “despicable act”. Sotloff’s family issued a statement saying it believed he had been killed.

State Department spokeswoman Jen Psaki said the US had seen “reports of a video that purports to be of the murder” of Sotloff.

“The intelligence community will work as quickly as possible to determine its authenticity,” she said. “If the video is genuine, we are sickened by this brutal act taking the life of another innocent American citizen. Our hearts go out to the Sotloff family.”

In the video, seen by the Guardian, Sotloff appears on his knees – his hands tied behind his back – beside a man holding a serrated knife, reminiscent of the one shown in the Foley video.

The masked fighter indicated he was the same man who murdered Foley, saying: “I’m back, Obama, and I’m back because of your arrogant foreign policy toward the Islamic State.”

The video was recorded in desolate desert landscape that resembled the location in which Foley was murdered, although the latest video showed a flatter terrain. It is unclear when it was made, but it appears to be recent: the masked killer makes reference to recent US bombings in Iraq, including the airstrikes near Amerli that began on Saturday. Sotloff’s hair and his beard are also longer than in the Foley video, in which Sotloff also appeared.

In the latest video, Sotloff is dressed in an orange jumpsuit, apparently mimicking the ones once used at Guantánamo Bay, and is made to deliver a statement blaming Obama for his murder. The killer states: “You, Obama, have yet again, through your actions, killed yet another American citizen.” He then appears to slice Sotloff’s throat.

The black-masked killer again urges Obama to cease his bombing campaign in Iraq, and says that if the US does not cease bombing, “our knife will continue to strike the necks of your people”.

The warning is extended to governments that might join the the “evil” actions of the US against Isis. It is around this point that the video cuts to another kneeling captive, stated to be a British national.

British intelligence officials have been working urgently to try to identify Foley’s killer, reportedly using sophisticated voice recognition techniques to help narrow down a list of potential British jihadi suspects.

UK government sources said that an early analysis of the video suggested that the British man whose voice issued this threat was the same man heard on the video that depicted the murder of Foley.

A video purportedly showing US journalist Steven Sotloff kneeling next to an Islamic State fighter. Photograph: Reuters

Administration officials have discussed US hostages held by Isis in plural terms, but have never confirmed that Sotloff or any other specific person was intended to be freed.

However on Tuesday, in brief, off-the-cuff remarks delivered as news of the video was breaking, the White House press secretary Josh Earnest appeared to indicate Sotloff may have been among the hostages Washington tried to free.

“The United States has ... dedicated significant time and resources to try and rescue Mr Sotloff,” Earnest said.

By Monday, the US had conducted 123 airstrikes against Isis positions, artillery and vehicles since 8 August. The campaign has now extended to providing humanitarian relief to besieged Shia Turkmen in the Iraqi city of Amerli, a weekend expansion that underscored the Obama administration’s determination to continue its war against Isis, despite Foley’s killing and the threat to Sotloff’s life.

That airdrop of water and food was accompanied by US airstrikes on Isis positions around Amerli, said Rear Admiral John Kirby, the Pentagon spokesman. “Further airstrikes remain a possibility, but we believe the township is under the control of Iraqi and Kurdish forces,” Kirby said Tuesday.

Sotloff, 31, a freelance journalist from Miami, Florida, disappered near the Syrian-Turkish border in August last year while working for the Time and Foreign Policy magazines.

A spokesman for Sotloff’s family said: “The family knows of this horrific tragedy and is grieving privately. There will be no public comment from the family during this difficult time.” Last week Sotloff’s mother Shirley had pleaded with Abu Bakr al-Baghdadi, the leaser of the Islamic State, for the release of her son, whom she described as an innocent journalist.

She recorded the plea after the video of Foley’s killing last month was released. In that film, Foley’s killer said her son would be next in line if the militant group’s demands were not met.

Tuesday’s video followed a similar pattern. It finished with footage of the British hostage, in the same style of orange jumpsuit that both Foley and Sotloff were wearing, suggesting he was their next intended victim.

Finally, with the Briton still in shot, the masked killer states: “Back off and leave our people alone.” A caption identifies the man, although the foreign office in London requested that his name not be published at this time.

At Downing Street, Cameron denounced the apparent killing of Sotloff: “If verified, this is a despicable and barbaric murder. My thoughts and prayers are with Mr Sotloff’s family and friends tonight as they deal with this appalling and tragic situation.

“As I have said consistently over the last few weeks, Isil terrorists speak for no religion. They threaten Syrians, Iraqis, Americans and British people alike and make no distinction between Muslims, Christians or any other faith.”

Cameron will chair a meeting of the British government’s emergency response committee, Cobra, on Wednesday to consider responses to the killing and the threat to the British hostage. He has said that he believes the West faces a “generational struggle” against the extremist ideology of the Islamic State, even suggesting it to be a more dangerous force than al-Qaida.

The militant shown in the video has identified himself to hostages who were subsequently freed as “John”, and is believed to be the ringleader of three British jihadists thought to be the main guards of foreign nationals in Raqqa, a stronghold of Islamic State fighters.

One former hostage, who was held for a year in Raqqa, told the Guardian John is intelligent, educated and a devout believer in radical Islamic teachings. He and his two British friends were referred to as “the Beatles” by fellow hostages because of their nationality, the former captive said.

Sir Peter Westmacott, the British ambassador to the US, has said that the UK and US intelligence agencies are close to identifying John, using voice-recognition technology.